Procedure:

make sure you are in HBOOT mode (you should see “HBOOT” after the ROM information)

make sure the Bootloader is unlocked (you should see “UNLOCKED” on the top of the screen)

Select [FASTBOOT] using [volume up/down] to move and [power] to select (when you select it you should see FASTBOOT, instead of HBOOT)

connect the device to the pc via usb

check if the device is available by:
fastboot devices

(note that if the device is not there and you are pretty sure that the usb is connected. you may have driver problems. in that case i would recommend to install PdaNet from http://junefabrics.com/android/download.php, just complete the first part of its installation. it will be enough to get the drivers working)